Before the Trio
Andrew is Tucker Wells's brother (the guy who trained hellhounds to attack the senior prom) and tried to be known as the guy who sent winged monkey demons to attack the school play. Despite this, most people only looked at him as "Tucker's brother". It was never stated that Buffy dealt with this thread directly, but she did thwart Tucker's plans by killing his hellhounds. Warren went to the same college as Buffy, and had fashioned his "perfect woman", an android, which he left in his dormroom until her batteries drained. Buffy met Warren when the andriod was harming people in order to find Warren, and nearly killed his girlfriend. Jonathan was a social outcast from Sunnydale high who had encountered Buffy several times, most notably when she talked him down from killing himself at school and when she had broken a spell which made the entire world believe Jonathan was an idol and icon.
The trio was seemingly formed mostly out of boredom when Andrew Wells, Warren Mears, and Jonathan Levinson got together with ambitions to use their abilities to gain money and power. Their first venture as the Trio was thwarted by Buffy when they hired a demon to help rob a bank that Buffy was at applying for a loan. After causing trouble with this first plan, the group decided that they were Buffy's arch nemeses, however they barely registered on Buffy's radar as more than a nuisance at first.
On one of their earliest missions against Buffy, the group sought to determine whose tactics and abilities were the most effective at screwing with Buffy's life. Warren attached a small device to Buffy which changed her perception of time, making the world a dangerous place until she discovered it. Andrew summoned a group of demons to attack Buffy during her first day of work at Xander's construction site. Jonathan cast a spell on her first day on the job at the Magic Box which made an experience with a relatively unreleasable customer repeat itself until Buffy resolved the situation to the customer's liking.
Later the trio accidentally turned Buffy invisible with an invisibility device which they planned on using for themselves. Buffy stopped them and finally learned who had been irritating her, but the group was able to escape.
Later, Jonathan used a spell to create an orb which would allow the three to control the mind of anyone they pleased. Warren used this device first on his ex-girlfriend (the one that was nearly killed by his love android). He intended to make her his willing love slave permanently, however the spell didn't work and he accidentally murdered her to prevent her from escaping to inform the authorities. This marked a very dark turn for the group, and Warren continually sought to establish a more powerful role within the trio. Summoning demons which messed with a person's perception of time, the trio made Buffy believe that she accidentally killed the girl while battling demons. Distraught over what she believed she had done, she was about to turn herself in to the police when she realized it was Warren's ex that was dead and put together exactly what had happened.
Warren continued to invest himself into the Slayer's demise, and with Jonathan and Andrew's help acquired orbs which would grant the bearer super strength and near invulnerability. Jonathan was upset with where the group was heading, and Andrew realized that Jonathan's behavior was similar to what he had displayed as a child before tattling to his parents. Andrew and Warren plotted against Jonathan and arranged an armored car heist which would rid them of Jonathan. While attacking the armored car with his new strength and invulnerability, Warren was stopped by Buffy. Warren and Buffy battled until Jonathan tipped her off to destroy the orbs on his belt. She did so, rendering Warren powerless against her. Warren was able to escape with a jet-pack he had concealed under his clothing. Andrew revealed a similar device (identifying to Jonathan that he had been betrayed), but while trying to escape he ran himself into an awning, knocking himself unconscious. Buffy took the pair to jail, where Jonathan and Andrew would bicker about whose fault it was. Andrew held out hope that Warren would come back to break them out (or perhaps just him).
The Trio's Demise
Warren, insane with rage, purchased a handgun to kill Buffy with. He went to Buffy's house with the intent to kill her for embarrassing him. Warren was able to get off several shots, hospitalizing Buffy and killing Willow's girlfriend Tara who was nearby. This drove Willow insane. She went to the Magic Box and drained all of the dark magics from the more dangerous books they kept, and went after the trio. Willow met up with Buffy and Xander at the hospital where Buffy was receiving emergency surgery, and drew the bullets out of Buffy, healing her. The three went after Warren who Willow was tracking to a bus that was heading out of town. Willow stopped the bus and destroyed the android that Warren had sent out as a decoy. Leaving Buffy and Xander, Willow left on her own for Warren.
She caught up with Warren in the woods where she held him captive, torturing him nearly to death. When she got bored with him she ripped his skin from his body, and he disappeared (falsely presumed to be dead).
Willow then went after Andrew and Jonathan who were still in police custody. Buffy, Xander, and Anya were barely able to get to Jonathan and Andrew in time to get them away from Willow. In Buffy's custody, Andrew and Jonathan were taken to the Magic Box, where Anya prepared a counter-spell to Willow's magic. When Willow arrived, Buffy and Anya were able to keep her busy long enough to get Andrew and Jonathan out with Xander and Dawn. When one of Willow's spells caught up to Xander, Andrew and Jonathan were able to slip away. The pair got into their van and drove to Mexico to hide while the Scooby gang disarmed Willow.